control of electric field stress in gas insulating busduct...

Control of Electric Field Stress in Gas Insulating

Busduct using Nano-Nitride Fillers

Hannah Monica Anoop1, G. V. Nagesh Kumar1, K Appala Naidu1,

D. Deepak Chowdary2 and B Venkateswara Rao3

1Vignans Institute of Information Technology, Visakhapatnam, India 2Dr. L. Bullaya College of Engineering for Women, Visakhapatnam, India

3V R Siddhartha Engineering College (Autonomous), Vijayawada, India

Abstract. Gas Insulated Substation (GIS) gains an ever increasing importance

due to the ever growing demand for electricity and energy density in

metropolitan cities. The insulation integrity of the spacer ensures the reliability

of GIS. So, it is of at most importance that the electric field distribution along

the spacers surface is simulated and an optimization of the same is done in

order to prevent the flashovers especially at the triple junction (TJ) formed by

SF6 gas, the spacer and the electrode. The distribution of electric field depends

considerably on the geometric shape of the spacer. Epoxy resin exhibits

excellent electrical, thermal and mechanical properties. This can be further

enhanced to a great extent by reinforcing epoxy with nano filler mixture as a

dielectric coating. The condition assessment can be done using Finite Element

Method, one of the proven methods for calculating the electric field density at

various points under consideration and the condition enhancement is done by

changing the filler concentration or by changing the thickness of the dielectric

coating. In this paper, the distribution of along the spacers surface is plotted,

the relative permittivity, breakdown voltage, thermal conductivity and

maximum electric field are calculated for a conical spacer are determined and

analyzed for different filler concentrations and different thickness of the

dielectric coating.

Keywords: Electric Field; Gas Insulated Systems; Nano Nitrides; Polymeric

Insulators

1 Introduction

Gas Insulated Systems (GIS) which are highly reliable, compact and pollution-free

have a potential to lead to a breakthrough in the Indian power scenario whose major

challenges are rapid urbanization, increasing energy density and scarcity of land. The

reliability of Gas Insulated Substations is of at most importance as any small failure

can elevate to a major problem in the grid it is connected to resulting in blackouts in

the power system. A survey of the failures in Indian GIS has shown that almost 30%

of the failures are due to selection of wrong materials, improper material substitutions

and material failures [1]. The condition assessment and enhancement of same is a

thrust area of the research and has drawn the attention of many researchers.

Advanced Science and Technology Letters Vol.147 (SMART DSC-2017), pp.1-7

http://dx.doi.org/10.14257/astl.2017.147.01

ISSN: 2287-1233 ASTL Copyright 2017 SERSC

As the compactness of GIS is increased, the electric field stress developed in the

Gas Insulated Busduct (GIB) comprising of the solid insulator called spacer, gaseous

insulator and the conductor, increases. A HV system experiences extreme conditions

which include high electric fields, high temperatures, mechanical stress and intense

radiations. From [2], the electric field stress developed at the ends of the solid

insulator increases at certain conditions leading to partial discharges weakening the

dielectric strength and early degradation. Under severe conditions, a complete

flashover occurs. It should have the capability to withstand not only regular voltages

but also over voltages caused by lightning, switching etc. Upgradation to ultra-high

voltage lines and extra-high voltage lines necessitates insulating materials which can

withstand high voltages, polarity reversal and space charge accumulation.

Generally, pure SF6 or mixture of SF6 and N2 is used at high pressure are used as

the gaseous insulator. The solid insulators called spacers which contain the

conductors are also used to support and separate various sections in GIB. Spacers

produce a complex dielectric field and intensify the electric field on the spacers

surface. The dielectric strength of SF6 is sensitive to maximum electric field. The

dielectric strength along the surface of the spacer is generally lower than that of

gaseous medium [3]. So the spacers should be designed such that, more or less a

uniform electric field distribution occurs along the spacers surface which will be

more reliable and flashover free. High operating temperatures and accumulation of

heat causes heating up of the equipment. It results in looseness between the devices

and consequent reduction of its lifespan. So the enhancement of the thermal properties

of the insulating materials is very important. Other factors like particle dispersion,

electric erosion, electrical treeing and interface properties greatly affect the

breakdown voltage. The future of the power systems lies in the progress of the

insulating materials with superior thermal, mechanical and electrical properties.

The development of polymers which conduct heat through vibration of atoms,

groups and chains, led to the synthetic materials like varnish, resins, impregnated

insulating fiber and composites [4] which had better insulating properties to be used

in even extreme conditions. In [5], the performance of the spacers in various shapes

like cone, smooth disc and corrugated disc has been reviewed. The intensification of

the local electric field which is a major problem has been considered in [6] in a cone-

type spacer fitted between the flanges in GIB. Various techniques have been

implemented to obtain improved insulating properties and uniform electric field but

with the limitation of a complex geometry of the spacer rendering it almost

impossible to manufacture.

The breakdown of dielectric occurs at submicron or nanoscale weak points like

interface between dielectric and electrode or other interface regions within the

dielectric. In 1994, Lewis introduced the concept of nano-dielectrics [7].

Investigations have proved that epoxy with nano-composites exhibit superior

electrical and mechanical properties when compared to pure epoxy resin and epoxy

resin with micro-fillers at low concentrations [8]. It was proved that the permittivity

depends greatly on the type and size of filler [9], combination of matrix and filler and

the smoothness of the samples [10]. In [11], it was shown that the epoxy

nanocomposites accumulate lesser charge compared to that of the clean epoxy resins.

From [12], it is shown that the charge dynamics are faster in epoxy nano-composites

Advanced Science and Technology Letters Vol.147 (SMART DSC-2017)

2 Copyright 2017 SERSC

and it is observed most evidently in case of negative charges. They exhibit a high

resistance towards partial discharges and electrical treeing and low dissipation factor.

Research has shown orderly arrangement of spherulite structures which prevents

the development of electric erosion helping the polymers to resist corona and partial

discharge [13-14].

The thermal conductivity of the dielectrics can be improved by adding

nanoparticles. By changing the amount, type and surface modification method the

thermal properties can be enhanced. Various fillers such as Al2O3, BN, AIN and

BNNT have been modified and added to different matrices, which include polyamide,

epoxy and silicone rubber.

The zone of interaction between the polymer matrix and nanoparticles is

considered as an independent area. When nanoparticles are in isolated dispersion, the

carriers are restrained in the interaction area. This results in the reduction of the

density of charge carriers as well as the mobility of the charge carriers. The thickness

of the interaction zone increases with an increase in the filler concentration which

greatly increases the mobility and density of the carriers. The interaction strength

between the polymer matrix and the nanoparticles greatly affects the thickness of the

interaction zone. The incorporation of nanofillers into the polymer matrix results in a

structural change of polymer caused by the polymer-nanofiller interaction. Using

inorganics nanofillers like aluminum-nitride and boron-nitride in polymeric matrices

reduces the cost, improves the fire resistance, mechanical characteristics like tensile

strength and permittivity.

In this paper, the distribution of electric field along the spacers surface coated with

dielectric coatings of nanonitrides with different concentrations is calculated using

Finite Element Method (FEM). The overall insulation integrity of GIB is determined.

2 Calculation of Relative Permitivity

The electric field in a given volume will be weakened when a material whose

dielectric constant is high is placed in it. Polyethylene can be placed between the

inner conductor and the outer enclosure in a coaxial cable. Epoxy/epoxy based

nanocomposites are preferred insulating materials for electrical applications for

bushings, GIS spacers etc. In epoxy nano composite, nanocomposites play a vital part

in the enhancement of the properties of epoxy because the permittivities of fillers are

high. Due to the higher individual permittivities of the fillers and on combining with

epoxy resin overall permittivity of the composite increases when compared to net

epoxy and epoxy micro composite. The filler loading can considered up to certain

extent based on the advantage of the interaction zone. It filler concentration is

increased to a high value which leads to over lapping of the interaction zone between

polymer matrix and filler due to which conductivity increases. The overlapping of the

nanoparticles in epoxy nanocomposite depends upon the rate of dispersion of

nanoparticles in the epoxy resin. The permittivity of a two phase dielectric satisfies

the Lichtenecker-Rother mixing rule which can be extended and written as shown in

equation (1)


Copyright 2017 SERSC 3

1 2 3cLog xLog yLog zLog (1)

where c is resultant composite permittivity, 1 , 2 , 3 are the permittivities of the filler

and epoxy and x and y, are the concentrations of filler and polymer.

3 Breakdown Voltage

For transmission and distribution of electric power three-phase common enclosure

GIB is used GIS. Inner surface of the bus duct is dielectric coated with epoxy

nanocomposites. To determine the breakdown voltage in terms of coating thickness

and permittivity can be written as

-1trv = v 1+

b d

(2)

where t is the thickness in m, V is the voltage applied, d is the gap and r is

the relative permittivity. The use of without surface treatment of nanofillers in epoxy

nanocomposite there is no changes in breakdown voltage. The breakdown voltage is

calculated at is 310-3.

4 Thermal Conductivites

The nanofillers have the individual thermal conductivity values are high. The epoxy

resin thermal conductivity is 0.168w/m.k. The thermal properties of epoxy resin,

nanocomposites are added to the matrix. The thermal conductivity is predicted from

the Agari and Uno model;

(3)

where, c1 and c2 are the adjustable constants, kf is the thermal conductivity of the

filler, km is the thermal conductivity of polymer matrix, kc is the resultant thermal

conductivity, is the volume fraction of the filler additives.

(4)

where w is weight fraction, f is the density of the filler,

m is the density of polymer matrix. The weight percentage of nanofillers increase then thermal

conductivity of epoxy increases well.

c 1 f 1 mLogk = .c .Logk + 1- .Log c .k

1

w

fw w

m



5 Results and Discussions

The filler concentrations of aluminium nitride are varied and variation of various

parameters like resultant permittivity, break down voltage and maximum electric field

are calculated and presented in Table 1. As the filler concentrations of aluminium

nitride are increased there is a gradual increase from 3.61 (for filler concentration of

0.2) to 4.52 (for filler concentration of 10). The break down voltages are calculated

for various filler concentrations of aluminium nitride. It is observed that with the

decrease in the thickness, there is an increase in the breakdown voltage. However,

there is a minor change in the maximum Electric field from 1.14 to 1.16. As the

filler concentrations of Boron nitride are increased there is a gradual increase from

3.60 (for filler concentration of 0.2) to 4.14 (for filler concentration of 10). The Break

down voltages is calculated for various filler concentrations of Boron nitride. It is

observed that with the decrease in the thickness, there is an increase in the breakdown

voltage. However, there is a minor change in the maximum Electric field from 1.14 to

1.1548.

Table 1. Variation of Various Parameters with Aluminium Nitride Filler Concentration

Filler

Concentratio

n

Resultant

Permitivity

Breakdown

Voltage at

40m

Breakdown

Voltage at

130 m

Maximum

Electric

Field

0.2 3.61 1059.60 1057.25 1.14

0.4 3.63 1059.60 1057.25 1.1415

0.6 3.64 1059.59 1057.22 1.142

0.8 3.66 1059.59 1057.20 1.1423

2 3.76 1059.56 1057.10 1.143

4 3.94 1059.51 1056.93 1.145

6 4.13 1059.45 1056.76 1.152

8 4.32 1059.40 1056.58 1.156

10 4.52 1059.34 1056.39 1.164

The variation of relative permittivity with filler concentration of aluminium nitride

and boron nitride are plotted in Fig 1. It is observed that there is a linear increase in

permittivity with increase in filler concentrations. Permittivity with aluminium nitride

filler concentration is more than that of boron nitride filler concentration.



Fig. 1. Plots between permittivity and filler concentration

6 Conclusion

The flashovers in critical areas which lead to complete breakdown of the insulators

can be prevented during the design by having a precise knowledge of the distribution

of the electric filed. The electrostatic field developed is greatly influenced by the

geometric shape of the electrode. The electric field distribution along the electrode

surface and the dielectric surface has to be carefully considered during the design and

optimization of the high voltage equipment. The model has been developed for a

single phase enclosure with an objective to obtain a quasi-stationary electric field

distribution. Nano composites enhanced the electrical and thermal strengths of

insulating materials in a gas insulated bus duct. Nitrides like aluminum nitride and

boron nitride enhanced the break down voltage and electric field.

References

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